Bell inequalities in 2-2 scattering

TL;DR

This paper investigates Bell inequalities in 2-2 scattering processes involving photons, gravitons, fermions, and pions, revealing implications for quantum entanglement, axion physics, quantum gravity, and the S-matrix bootstrap approach.

Contribution

It introduces a comprehensive analysis of Bell violations across various particle scatterings, connecting quantum entanglement with fundamental physics constraints and recent S-matrix data.

Findings

Bell violation in low-energy QED scattering except for a small transverse region.

Inclusion of axions removes fine-tuning issues and constrains axion parameters.

Bell violation in graviton scattering aligns with the Weak Gravity Conjecture.

Abstract

We consider Bell inequalities in 2-2 scattering of photons, gravitons, fermions and pions. We choose measurement settings that give maximum Bell violation for maximally entangled states and calculate the relevant Bell inequalities for these processes. For photon scattering at low energies, QED exhibits Bell violation for all scattering angles except for a small transverse region. This leads to a fine-tuning problem. Incorporating a light axion/axion-like particle (ALP) removes the fine-tuning problem and constrains the axion-coupling--axion-mass parameters. Allowing for graviton exchange and demanding Bell violation in photon scattering, we find that the Weak Gravity Conjecture is satisfied. Quantum gravity effect on axion coupling is discussed. For 2-2 graviton scattering, we find that CEMZ bounds allow for at most small Bell violations. Restriction on the Weinberg angle is found by demanding Bell violation in Bhabha scattering. We use recent S-matrix bootstrap data for pions and photons to study the Bell parameter in the space of allowed S-matrices. In the photon case, we study the Bell parameters as a function of energy and find support for the EFT observations. We discuss Bell parameter for pion S-matrices, which are qutrits. For pions, we find that there is a minimization of a suitable Bell parameter for S-matrices which exhibit Regge behaviour.